This invention relates to tensioners used with chain drives in automotive timing applications and more particularly to a hydraulic tensioner having a blade-type tensioner arm.
Tensioning devices are used as a control device for a power transmission chain as the chain travels between a plurality of sprockets. Generally, it is important to impart and maintain a certain degree of tension to the chain to prevent noises or slippage. Prevention of slippage is especially important in the case of a chain driven camshaft in an internal combustion engine because slippage may alter the camshaft timing by several degrees, possibly causing damage. In the harsh environment in which an internal combustion engine operates, chain tension can vary between excessively high or low levels as a result of the wide variations in temperature and differences between the coefficients of linear expansion among the various parts of the engine, including the chain and the tensioner. Camshaft and crankshaft induced torsional vibrations cause chain tension to vary considerably. This tension variation results in chain elongation. Moreover, wear of the chain components during prolonged use can cause elongation of the chain that results in a decrease in the tension of the chain.
Generally, blade-type chain tensioners use a blade spring interlocked under tension with a single shoe to provide tension to a chain. The blade spring is arcuate in shape and the shoe is relatively flat. The shoe is constructed from a semi-rigid material which will deform or "creep" upon experiencing a load at a high temperature. The blade spring is flattened to correspond to the shape of the shoe and then interlocked with it. Because the semi-rigid shoe prevents the blade spring from returning to its original more arcuate shape, the blade spring applies a load to the shoe.
During operation, as the heat from the engine causes the temperature of the shoe to increase and become less rigid, the load from the blade spring causes the shoe to deform to a more arcuate shape. Through such deformation, tension is provided to a chain. The chain tensioner assembly is positioned along a free length of the chain between the sprockets. As the blade spring forces the shoe into a more arcuate shape, the apex of the shoe extends farther into the span of chain thereby increasing chain tension.
Typical blade-type chain tensioners have interlocked a blade spring to a single shoe. For example, U.S. Pat. No. 3,490,302, to Turner et al., U.S. Pat. No. 4,921,472, to Young et al., and U.S. Pat. No. 5,055,088, to Cradduck et al., each disclose a blade-type tensioner having a blade spring mechanically interlocked with a shoe. U.S. Pat. No. 5,266,066, to White discloses a blade-type chain tensioner in which a blade spring is constructed from a simple rectangular metal band formed into an arcuate shape and interlocked within a pocket in a shoe to provide a load to the shoe.
U.S. Pat. No. 5,462,493 addresses the limited arcuate range and oscillation problems of these single blade tensioners by providing two chain tensioner shoes in an overlapping configuration. One shoe imparts tension to the chain, while the other shoe damps the movement of the first shoe to reduce the oscillations. The overlapping shoe configuration also allows a greater range of arcuate movement so that the chain tensioner is able to meet the greater transverse movement associated with a longer center length timing chain.
U.S. Pat. No. 5,653,652 provides for the two chain tensioner arms in an overlapping configuration, as referred to in U.S. Pat. No. 5,462,493, but also adds a hydraulic tensioner against the arms in order to provide a system with greater range of tensioning.
One problem associated with the tensioning system of U.S. Pat. No. 5,653,652, in certain types of timing chain configurations, is the set geometry provided by the system throughout its range of operation. Maintenance of contact between the chain and the tensioner arm is essential to maintain control of a chain system. In systems of fixed geometry, the contact area between these two elements (chain and tensioner arm) changes depending on the position or articulation of the tensioner arm.
On chain systems with particularly short center distances, the fixed geometry is often such that the contact patch (or portion of the tensioner arm contacted by the chain) must be very short in any position of the tensioner so that the contact can be maintained throughout the life of the tensioner system as the patch moves along the arm due to wear over the life of the system. The present invention attempts to provide a longer contact patch throughout the life of the tensioner system since the tensioner arm can conform to the geometry of the chain strand at various positions.
Another problem faced by systems of U.S. Pat. No. 5,653,652, in certain timing system configurations, arises due to slap or contact as the chain impacts the tensioner arm. Slap by the chain against the arm can cause pitting and material removal from the arm, which quickly leads to failure of the arm surface. The present invention provides a flexible arm that travels with and conforms to the path of the chain to minimize or eliminate impact in the form of slap or contact.